1. Field of the Invention
This invention relates, in general, to a method for material purification and, specifically, to a method for purification of metal halides.
2. Description of the Prior Art
Currently there are several schemes available for the ultra-purification of metal halides, especially metal fluorides used to make heavy metal fluoride glasses. However, current methods have not yet obtained the purity required for some application. For example, the purity levels thus far attained for metal halides fall short of those necessary for attainment of the theoretical minimum loss in a fluoride fiber of 0.01 dB/Km.
Wet chemical processing has limitations in the degree of purification because of recontamination from the background levels of contaminants present in the processing chemicals. Vapor preparation techniques are limited by low or non-existent thermodynamic driving force for removal of the contamination. The reactive atmosphere process (RAP) diminishes the hydroxyl content but has little effect on other contaminants.
Sublimation and distillation are separation techniques which have been used to purify heavy metal halides such as zirconium and hafnium tetrafluorides. These separation techniques are satisfactory for removing the majority of cation impurities found in commercially received material, such as the alkaline earth and rare earth impurities. However, sublimation and/or distillation have been only partially effective in removing iron impurities due to the relatively high vapor pressure of Fe.sup.+3.
U.S. Pat. No. 4,578,252 discloses a method for preparing ultra-pure pure metal tetrafluorides. The disclosed method removes transition metal impurities from zirconium and hafnium tetrafluorides by converting iron cations to iron metal during distillation and sublimation. Using electromotive series displacement to displace the iron with zirconium, this method could produce metal tetrafluorides having iron impurity concentrations of below 1 ppm.
U.S. Pat. No. 4,741,752 disclosed a process for optical fiber in which Fe(II), which absorbs light of certain wavelength is converted to Fe(III), which absorbs light of different wavelengths. The conversion occurs in a melt of a halide glass composition by reaction with dry oxygen.
At present, the primary method of producing metal fluorides at the ppb purity level is via ion exchange. This method, while capable of producing material of high purity in large quantities, cannot achieve ultra-purification without encountering significantly increased cost and time and decreased efficiency.